1. Field of the Invention
The present invention relates to an emitter-follower type bias circuit supplying a bias voltage to a base of an amplification transistor, and in particular to an emitter-follower type bias circuit which can operate at a low reference voltage.
2. Background Art
Currently, GaAs-HBT power amplifiers are increasingly being used for CDMA-based cellular phones, wireless LAN or the like (e.g., see Japanese Patent Laid-Open No. 2004-343244).
Conventional power amplifiers receive a reference voltage from outside as input. Since the reference voltage determines an idle current of each power amplifier, the reference voltage needs to be kept constant with high accuracy (e.g., on the order of 2.85 V±0.1 V) irrespective of any variation in a power supply voltage. There is a demand in recent years that a reference voltage should be generated inside the power amplifier. In this case, the reference voltage is generated inside the power amplifier in response to an enable signal (a digital signal to turn ON/OFF the power amplifier) given from outside.
Furthermore, there is also a demand in recent years that an enable signal should be generated at a lower voltage. That is, power amplifiers are conventionally operated with an enable signal on the order of 2.6 V, but it is only recently that power amplifiers are required to be operated with an enable signal on the order of 1.4 V.